Changing a user group membership is the same as reassigning a user to a new group.
1. click the Gear icon in the top right corner
2. select user management
3. expand the groups and click on a group you want to edit
4. Search for the user or select from the list of users you want to add to the group. To remove a user from the group, click on the user in the Assigned Users section and the user will be removed from the group. The same user can be added to multiple groups. The user roles will be the cross product of all the groups.
Then save.